1. Field of the Invention
The present invention relates to a solid electrolyte battery including a cathode having at least a cathode active material and a solid electrolyte and an anode.
2. Description of Related Art
In recent years, with the rapid development of portable electronic devices such as video cameras with video tape recorders, portable telephones, lap-top computers and so on, it has been demanded to more improve the performance of electrochemical devices as means for these electronic devices.
As the electrochemical device of a secondary battery, there has been hitherto employed liquid electrolyte (electrolyte solution) obtained by dissolving electrolyte salt in, for instance, water or organic solvent as a material for bearing an ionic conduction.
However, since liquid may possibly leak in the electrochemical device using the electrolyte solution, a sealing property needs to be ensured by using a metallic vessel. Therefore, the electrochemical device using the electrolyte solution includes various inconveniences that the weight becomes large, a sealing step is troublesome and a configuration is hardly freely designed.
Thus, as the material which bears the ionic conductivity of the electrochemical device, the study of, what is called a solid electrolyte made of a solid ionic conductor has been vigorously carried out.
Since the solid electrolyte does not have such an anxiety as to leak the liquid, the solid electrolyte is employed for the electrochemical device so that the sealing step for preventing the leakage of liquid can be simplified and the weight of the electrochemical device can be decreased. Further, when a solid polymer electrolyte including a polymer compound is employed as the solid electrolyte, since a polymer has an excellent film moldability, the electrochemical device whose configuration is extremely freely selected can be advantageously manufactured.
The solid electrolyte, particularly, the solid polymer electrolyte ordinarily comprises a matrix polymer and electrolyte salt capable of ionic dissociation. The matrix polymer has an ionic dissociation performance including both a function for solidifying the solid ionic conductor and a function as a solid solvent for the electrolyte salt.
As for such a solid electrolyte, for example, Armond et al. of Grenoble University (France) reported in 1978 that the ionic conductivity of about 1×10−7S/cm in the solution having lithium perchlorate dissolved in polyethylene oxide was obtained. Since then, a variety of polymer materials mainly including polymers having polyether bonds have been eagerly examined.
What is called, a solid polymer electrolyte battery has such a solid polymer electrolyte arranged between a cathode and an anode.
Generally, electrodes employed in a so-called lithium-ion secondary battery include, for instance, a coating type cathode, a coating type anode, a separator and electrolyte solution.
For instance, the coating type cathode is formed in such a manner that a cathode active material including LiCoO2, a conducting assistant including graphite and a binding agent including polyvinylidene fluoride (abbreviated it as PVdF, hereinafter) are mixed together in the prescribed mixing ratio, the obtained mixture is dispersed in a solvent to obtain a composite mixture and the composite mixture is applied to a cathode current collector composed of an aluminum foil and the applied composite mixture is dried. Further, the anode is formed in such a manner that graphite and a binding agent including PVdF are mixed together in the prescribed mixing ratio, the obtained mixture is dispersed in a solvent to obtain a composite mixture, the composite mixture is applied on an anode current collector made of a copper foil and the applied composite mixture is dried.
In the cathode of the lithium-ion secondary battery, an electron conducting path is formed by the cathode current collector and the conducting assistant. The electrolyte solution permeates through the electrode so that an ion conducting path is formed. In the anode, an electron conducting path and an ion conducting path are formed in the same manner as that described above.
On the other hand, the electrodes of the above-described solid polymer electrolyte battery do not include liquid components and is composed of powder. Accordingly, the electron conducting paths of the electrodes of the solid polymer electrolyte battery are composed of current collectors and conducting assistants in a similar manner to that of a liquid type battery such as the above-described lithium-ion secondary battery.
However, since the electrolyte of the solid polymer electrolyte battery is solid-state, it is impossible to employ a method for penetrating electrolyte solution into the inner parts of electrodes as in the case of the lithium-ion secondary battery. Therefore, in the solid polymer electrolyte battery, ionic conducting paths are hardly formed and an internal resistance is seriously high. When the thickness of coating on the electrodes is more increased in order to increase a battery capacity, the above-described tendency becomes the more outstanding.
Thus, in order to overcome the above-described problem, is proposed an idea that the above-described solid polymer electrolyte is used as a binding agent for a composite mixture so as to bear both a function as a binding agent of a current collector, an active material and a conducting assistant and a function as an ionic conducting path.
The solid polymer electrolyte which contributes to the ion conducting paths in the electrodes needs to have a high ionic conductivity like, for instance, an SPE (Solid Polymer Electrolyte: solid electrolyte film as a separator).
It may be said that an amorphous polymer having a low glass transition point is generally suitably employed as the solid polymer electrolyte from the viewpoint of obtaining a high ionic conductivity. However, such an amorphous polymer has original properties that it is very soft and its melting point is low, so that the amorphous polymer is inferior in its function as a binding agent.
Therefore, when the amorphous polymer is used as the solid polymer electrolyte, a larger amount of amorphous polymer needs to be added to a composite mixture than a case in which the binding agent such as PVdF employed in, for instance, a liquid type battery is employed in order to satisfy a separation strength or the like required for electrodes. As a result, since the amount of an active material in the composite mixture is relatively decreased, there is a fear that a battery utilization factor, and further, a battery capacity may be deteriorated.
For improving the separation strength, it is proposed that the solid polymer electrolyte serving as the binding agent is chemically bridged. However, according to this method, a step for manufacturing a battery is inconveniently troublesome and an ionic conductivity is disadvantageously lowered due to the chemical bridging.